1. Field of the Invention
The present invention relates to multispeed, selectively shiftable multiple axle assemblies such as tandem axle assemblies or 4.times.4 and/or 6.times.6 axle assemblies having a selectively lockable interaxle differential mechanism and more particularly, to multispeed multiple drive axle assemblies having interaxle differential lockups wherein engagement of the interaxle differential lockup will automatically prevent shifting of the drive axle assembly.
2. Description of the Prior Art
Multispeed, selectively shiftable drive axles, and the mechanisms and controls for selectively shifting same are well known in the prior art as may be seen by reference to U.S. Pat. Nos. 2,462,779 and 2,948,370, both hereby incorporated by reference.
Multiple drive axle assemblies, such as tandem axles, having interaxle differentials, also called power dividers, with selectively engaged interaxle differential lockups, and the mechanisms and controls for selectively locking the interaxle differentials, are well known in the prior art as may be seen by reference to U.S. Pat. Nos. 3,000,456 and 3,388,760, both hereby incorporated by reference.
Multispeed tandem axles having lockable interaxle differentials with separate controls for selectively shifting the axles and engaging the interaxle differential, and controls for limiting shifting of the axles in response to sensing engagement of the interaxle differential lockup are known in the prior art as may be seen by reference to U.S. Pat. No. 3,368,638 hereby incorporated by reference.
As is known, it is desirable to prevent shifting of a multiple axle assembly when the interaxle differential is locked up (i.e. not free to differentiate). This is because if the axles do not shift simultaneously, the individual drive axles, such as for example the front-rear axle and rear-rear axle of a tandem axle assembly, may momentarily be engaged in different speeds (i.e. different ratios) and if the interaxle differential is not free to differentiate under such conditions, damage is likely to occur.
The problem has been recognized in the past and control systems such as illustrated in above-mentioned U.S. Pat. No. 3,368,638 have been provided. However, while generally satisfactory, the prior art systems have not been totally satisfactory as such systems typically allow the interaxle differential to be engaged in only one of the selected speeds. For example, in the system illustrated in U.S. Pat. No. 3,368,638, engagement of the interaxle differential automatically vented the axle shift cylinders causing the spring biased pistons to shift all of the axles to low speed. Also, as engagement of the interaxle differential caused a shifting to low speed, and as a "torque lock" at one of the axle assemblies and/or different length air and/or hydraulic lines might cause nonsimultaneous shifting of the axles, damage could result. Additionally, as the prior art systems typically responded to an engagement of the interaxle differential by venting of the air or hydraulic actuated axle shift control mechanism, rather than by a positive lock out of the axle shift selector switch, driver abuse by preselection of an axle shift was possible.